Pliocene rhinocerotidae (mammalia) from hadar and dikika (lower Awash, Ethiopia), and a revision of the origin of modern African rhinos

Fossil representatives of the two extant African rhinoceros lineages, Ceratotherium and Diceros, co-occur in the Pliocene deposits of the Hadar Formation, Ethiopia. Both arose, in turn, from Ceratotherium neumayri of the late Miocene. The first of these Pliocene species, Ceratotherium mauritanicum, can be distinguished from the living C. simum, to which it probably gave rise in the earliest Pleistocene, by its less plagiolophodont cheek teeth. The second, Diceros praecox, is closely related to D. bicornis, although many specimens, including the type, were previously referred to Ceratotherium. The teeth of D. praecox are little changed relative to those of its Miocene ancestor, but its skull displays the apomorphic rearrangements typical of D. bicornis, suggesting increased browsing specialization. The split between the two lineages probably indicates ecological divergence and character displacement between browsing versus grazing specializations.